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I was recently reading Jack R. Woods' response on this forum: Could the James Webb Space Telescope detect biosignals on exoplanets?

In an ideal situation (say looking at absorption lines of a super-earth) there are many "bio-signatures", but one of the easiest to detect would be an ozone line in the infrared. By itself, this would not be proof, even though there would need to be a constant replenishing source of O2 in the atmosphere to maintain the O3. If methane could also be found we could rightly get VERY excited since methane and oxygen don't co-exist very well.

Question: Why wouldn't O3 detection be proof? Is there any non-biological process - already observed or theoretical - capable of generating O2 on a planetary scale with sufficient constancy to maintain an O3 layer?

Apparently there are theoretical processes that could actually generate O2 in the absence of life. I just don't know if it's proven: Titania may produce abiotic oxygen atmospheres on habitable exoplanets | Scientific Reports

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    $\begingroup$ relevant paper iopscience.iop.org/article/10.3847/1538-4357/aadd9b and probably the citations therefrom $\endgroup$
    – James K
    Dec 29, 2021 at 21:54
  • $\begingroup$ You have to be very careful with statements like that. Besides going there and seeing the life under a microscope, there is not ever going to be an absolute proof of life on an exoplanet. This won't stop however the claims of chemical disequilibrium == life. The research literature of the recent years (e.g. ui.adsabs.harvard.edu/abs/2021JGRE..12606711L/abstract) constantly produces new geochemical pathways to produce biomarkers without biological action. So, you would rather have to ask "Why would $O_3$ detection be proof? How do you know it's not a false positive?" $\endgroup$
    – AtmosphericPrisonEscape
    Dec 30, 2021 at 12:31
  • $\begingroup$ @AtmosphericPrisonEscape Yes, yes, my question was exactly whether we already knew about abiotic pathways capable of producing molecules that could be interpreted as "biosignals". And, from what I found, and also from what you sent, they exist, and they are even many; very interesting ^^ $\endgroup$
    – Octupos
    Dec 30, 2021 at 14:01

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